This invention relates generally to oscillators and, more particularly, to crystal controlled overtone oscillator circuits having means for assuring that the oscillator operates at a desired overtone frequency.
Overtone oscillators are well known in the prior art. The prior art overtone oscillators usually have adjustable inductances in the feedback path to prevent oscillation at lower overtone frequency and achieve the additional phase shift at the overtone frequency. See for example Irwin et al U.S. Pat. No. 3,875,533. The use of the inductance in the feedback introduces a temperature drift since a typical adjustable coil drifts about 400 ppm (parts per million) per degree celsius. It is therefore desirable to provide an overtone oscillator without the use of this feedback coil.
Although overtone oscillators of the negative resistance type as described in U.S. Pat. No. 3,512,107 of Miyake et al do not use an inductance, such circuits operate on the negative resistance principle and only at frequencies below 20 MHz with a narrow operating frequency range. The negative resistance type oscillator is very sensitive to multiple oscillation or frequency jumping between fundamental and overtone modes and/or at crystal spurious frequencies.